The invention is based on an apparatus for controlling the composition of the operating mixture to be introduced into the combustion chambers of internal combustion engines. In an apparatus of this kind, in accordance with the closed-loop control principle realized therein, the fresh-air intake cross section is closed by the throttle valve upon shutdown of the engine, and the exhaust recirculation valve is opened. Upon starting of the engine, an excess quantity of fuel is injected in a known fashion; this means that at this moment, because of the closed fresh-air intake cross section, there is insufficient air available for combustion in this fuel-guided, closed-loop control system. This causes problems in starting. But even after starting the engine, that is, during the warm-up phase, conditions for preparation of the fuel injected into the combustion chambers are still poor, so that recirculated exhaust gas hinders combustion. The effect is that so-called blue smoke appears in the engine exhaust, and the engine runs unevenly.